The present invention relates to an exhaust gas recirculation system for purifying the exhaust gases emitted from an internal combustion engine.
In any exhaust gas recirculation systems, it is a very important and critical problem how to determine the ratio of the flow rate of the exhaust gases to be recirculated to the quantity of intake air. (The above ratio will be referred to as "EGR ratio" for brevity hereinafter in this specification and the above flow rate, as "EGR quantity".) There has been devised and demonstrated a prior art exhaust gas recirculation system wherein in order to maintain EGR ratio constant all the time, a pressure chamber is provided in an exhaust gas recirculation passage in such a way that the pressure in the pressure chamber may be maintained nearly at the atmospheric pressure and the exhaust gases are forced into the pressure chamber through a restriction. This system is based upon the observed fact that the exhaust gas pressure is nearly square of the quantity of intake air. However, it has recently become well known to those skilled in the art that in order to improve the efficiency of the purification of exhaust gases as well as the riding quality, when the quantity of intake air remains constant, the higher the negative pressure in the intake pipe (to be referred to as "the negative intake pressure" for brevity in this specification), the smaller EGR ratio must be made. To this end, there has been devised and demonstrated and exhaust gas recirculation system wherein the opening of the restriction through which the exhaust gases are forced into the pressure chamber is varied in response to the negative intake pressure. More particularly, the variable restriction is controlled by a plunger which in turn is controlled in response to the negative intake pressure. However, this system gives rise to a problem of sealing the plunger, a problem of high fabrication cost and a problem of durableness of the variable restriction in the exhaust gases which are high in temperature and contain strong acids and carbon particles which cause rapid wear and abrasion of the variable restriction. Furthermore as the carbon particles deposit upon the variable restriction, its response to the variations in the negative intake pressure is varied, but so far there has not been proposed any successful method for correcting such variation.
A further problem of the prior art exhaust gas recirculation systems is that because of the high resistance to the flow of exhaust gases encountered in the exhaust gas recirculation passage, a satisfactory EGR quantity cannot be obtained with the exhaust gas pressure is weak. Furthermore, when the engine is decelerated so that the negative intake pressure is consideranbly increased, the exhaust gas recirculation cannot be interrupted.
In order to vary EGR ratio in response to the variations in load on engine, there have been devised and demonstrated various exhaust gas recirculation systems. In one system (I), a negative pressure created in a venturi in an intake pipe (to be referred to as "the venturi negative pressure" for brevity hereinafter in this specification) is amplified in response to the instant load on engine, and the pressure in the pressure chamber is maintained at a pressure nearly equal to the amplified venturi negative pressure. In another system (II), the pressure in the pressure chamber is maintained at a pressure nearly equal to a negative pressure at a port formed adjacent to the position where a throttle valve is completely closed. Both systems (I) and (II) have a common defect that the pressure in the pressure chamber cannot be made lower than the negative intake pressure. As a result, the system (I) cannot use a high amplification factor for the amplification of the venturi negative pressure in the case of a heavy load. In the system (II) the throttle valve is caused to open at heavy load so that the negative pressure at the port will not respond to the load on the engine. Thus it has been extremely difficult to maintain a high EGR ratio even at heavy loads while varying EGR ratio fully in response to the variations in load on the engine.